The productivity of oil and gas wells can be improved by increasing the area of communication within a selected producing zone. The drainage area can be increased by hydraulic fracturing of the producing zone to provide fractures and channels emanating from the well base area into the contiguous subterranean formations.
The hydraulic fracturing process is accomplished by rapid pumping of an aqueous fluid medium down a well which penetrates the subterranean formation where fracturing is desired. The rapid pumping of the aqueous fluid creates a hydrostatic pressure which energizes splitting forces in the confined zone. Pressures as high as 10,000 psi are employed to effect formation fracturing.
As cracks and channels are formed, a propping agent which is suspended in the high viscosity hydraulic fluid penetrates the newly created fissures and becomes lodged therein. The function of the propping agent is to support the fractures in an open position as a conduit for the flow of fluids such as oil, gas or water through the fractured zone. Various non-compressible materials are employed as proppants. These include sand, rounded walnut shells, glass beads, aluminum spheres, and the like. U.S. Pat. No. 3,708,560 proposes polyester resin prills as a proppant in hydraulic fracturing fluid systems.
After a fracturing operation has been completed and the propping agent has been deposited, the hydrostatic pressure is released, the flow of fluid is reversed, and the hydraulic fracturing fluid is withdrawn.
Hence, the hydraulic fracturing fluid composition functions to force fracturing under hydrostatic pressure, and it serves to transport the suspension of propping agent into the porous subterranean formations. The hydraulic well-treating fluid medium must exhibit advantageous viscosity and particulate solids transport properties.
Hydraulic fracturing fluids commonly employ a viscosity incresing agent such as a water-soluble polymer to improve the flow characteristics and the suspending ability of the fluids. Carboxymethylcellulose, polyacrylamide, polyvinylpyrrolidone, guar, tragacanth, Irish moss, modified starch, and the like, have been employed as water-soluble or water-dispersible additives to provide pumpable gels having solids suspending properties. U.S. Pat. Nos. 3,483,121; 3,757,864; 3,779,914; 3,818,998; 3,900,069; 3,971,440; and references cited therein, describe hydraulic well-treating compositions which contain novel polymeric friction-reducing gelling components.
Hydraulic fracturing fluids which contain a natural gum, such as unmodified polygalactomannan gum, commonly include a breaker additive to effect a delayed action thinning out of the fluid medium. This facilitates the subsequent removal of the hydraulic fracturing fluid composition from the fractured formation.
Breaker additives include enzymes which under the well-fracturing conditions in a delayed manner autonomously reduce the hydraulic fluid viscosity by degrading the natural gum incorporated as a gelling agent. U.S. Pat. Nos. 2,681,704; 2,801,218; 3,615,794; 3,684,710; and references cited therein, describe new enzyme compositions, such as mannan depolymerase which is derived from a microbiologic source.
The disadvantage of employing natural gum as a gelling agent in hydraulic fracturing fluids is the lessening of fluid loss control caused by plugging of apertures which reduces the permeability of a fractured formation. In the case where a natural resin such as guar gum is employed as a gelling agent in combination with an enzyme breaker additive in a hydraulic fracturing fluid, the hydraulic system is unsatisfactory because the hydrolysis residue of the natural gum is present in sufficient quantity to create a plugging effect and reduces the permeability of a fractured formation.
Accordingly, it is a main object of the present invention to provide natural gum derivatives having improved viscosity and suspending properties for application as gelling agents in hydraulic well-treating fluid compositions.
It is another object of this invention to provide hydraulic well-treating fluid compositions containing a natural gum derivative in combination with a breaker additive.
It is a further object of this invention to provide hydraulic well-fracturing fluid compositions containing a polygalactomannan ether derivative in combination with an enzyme as a breaker additive, which are characterized by a low yield of hydrolysis residue.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.